Glucagon-like peptide-1 (GLP-1) is an important glucoincretin hormone secreted from intestinal L cells in response to nutrient ingestion. GLP-1 functions to regulate plasma glucose levels via various independent mechanisms, making it an ideal candidate for treatment of diabetes, and possibly useful in the pharmacotherapy of obesity.
The biologically active forms of GLP-1 possess multiple functions in vivo, including enhancement of glucose-dependent insulin secretion, stimulation of proinsulin gene expression, and suppression of glucagon secretion and gastric emptying. GLP-1 also enhances insulin sensitivity, induces β cell differentiation and proliferation, decreases caloric intake, and increases satiety.
The mature, active form of GLP-1 is a 30 amino acid derivative of proglucagon, a 160 amino acid prohormone. GLP-1 is synthesized by post translational processing of proglucagon in intestinal L cells. Posttranslational processing of proglucagon gives rise to glucagon, GLP-1, GLP-2 and other peptide sequences, IP-1 and IP-2, of unknown function. The initial GLP-1 cleaved from proglucagon is further processed first by N-terminal cleavage to form a biologically active peptide (GLP-1(7-37)). GLP-1(7-37) is then C-terminally truncated and amidated to form the predominant biologically active species, GLP-1(7-36)amide.
GLP-1(7-36)amide has a very short half life in vivo. The plasma half life of GLP-1 is about 5 minutes, and the metabolic clearance rate is about 12-13 minutes. In circulation, the predominant form of GLP-1 is rapidly inactivated as a result of degradation by dipeptidyl-peptidase IV (see e.g., Deacon et al. (1995) Endocrinol. Metab. 80:952-957, and Hansen et al. (1999) Endocrinology 140:5356). GLP-1(7-36)amide is also susceptible to degradation by neutral endopeptidases, including NEP 24.11 (Sodman et al. (1995) Reg. Peptides 58:149-156).
The unique ability of GLP-1 to lower postprandial hyperglycemia via three independent and complementary mechanisms of action (increased insulin secretion, inhibition of glucagon release, and inhibition of gastrointestinal motility) are what make this peptide hormone an ideal candidate for the treatment of diabetes. Indeed, GLP-1 provides unprecedented advantages over any other pharmacological agent currently available. Unfortunately, despite its potential, there are serious limitations to the possible therapeutic use of GLP-1 in humans. The most serious limitation is the very short half life of GLP-1 in vivo. Even when administered subcutaneously, peak concentrations return to baseline within 90 minutes.
The therapeutic potential of GLP-1 and its very short half life have prompted the search for and discovery of analogs that may provide an extended GLP-1-like biological activity. Several analogs have been isolated from other species (Fehmann, H. C., et al. (1995) Endocrine Reviews 16:390-410, and Thorens B. et al. (1993) Diabetes 42:1678-1682), and mutant GLP-1 peptides resistant to degradation have been created (Xiao et al. (2001) Biochemistry 40:2860-2869).
Some GLP-1 analogs may show some promise as therapeutics. However, since GLP-1 peptide is a highly multifunctional protein, mutants and interspecies homologs may have unpredictable plieotropic effects. Indeed, Xiao et al. showed that some mutants exhibit altered biological activity independent of any changes in receptor binding activity. Thus, the biological activities of GLP-1 can be uncoupled from one another.
Diabetes, obesity and other disorders of sugar metabolism and glycemic control carry a very high price for the individual, as well as the society in terms of health, lost productivity and the loss of wages and financial output. Thus, there is clearly a need in the art for effective medications that facilitate glycemic control in the individual. A stabilized GLP-1 with increased half life in vivo could meet this need. Preferably a stabilized GLP-1 peptide would be very similar to the wild type protein, such that changes to biological activity, and hence possible side effects of therapy can be minimized. The present invention answers the need for stabilized GLP-1 molecules, thereby providing therapeutically effective GLP-1 peptides. Other objects and advantages will become apparent from the detailed description that follows.